Recent Developments


Preparation/Production Methods

Methods to improve the efficiency of the synthesis of Pigment Violet 23 have been the subject of several patents. The most recent describes the replacement of the traditional sulfonyl chlorides by tertiary amine N-oxides [12]. According to the authors the subsequently formed tertiary amines (the by-products of the oxida — tion/cyclization process) then may be recovered by distillation during the final stage of the synthesis. The recovered amines (e. g. pyridine) then may be reoxi­dized to the corresponding N-oxides and reused. This innovative recycling of the oxidant reportedly results in a significant reduction in the amount of waste gener­ated during product synthesis. In a further refinement, a catalytic quantity of a suitable quinone (2,3,5,6-tetrachloro-1,4-benzoquinone is preferred) is added to the reaction at the cyclization stage. Employing the optimum quantity in combi­nation with the appropriate tertiary amine N-oxide, an improvement in the effi­ciency of the cyclization/oxidation stage is effected, resulting in an almost quanti-

tative yield of product. All other things being equal, the impact of this technology improvement on economic and environmental factors is expected to be favorable.


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